It is known that optical fibers having the core doped with certain substances, e.g. the ions of rare earth, have characteristics of stimulated emission suitable for use as laser sources and as optical amplifiers.
In fact, such fibers can be supplied with a light energy at a specific wavelength which causes the atoms or ions of the dopant substance to reach an excited energy state, or pumping band, from which the atoms decay spontaneously in a very short time to a laser emission state where they remain for a relatively longer time.
When a fiber having a high number of atoms at the excited state in the emission level is transited by a light signal having a wavelength corresponding to such laser emission state, the signal causes the transition of the excited atoms to a lower level with an energy emission having the same wavelength as the signal. Thus, a fiber of this type can be used to obtain an amplification of an optical signal.
From the excited state, the atoms also decay spontaneously, and this generates an energy emission which constitutes a background noise signal which is superimposed on the stimulated emission corresponding to the amplified signal.
The light emission generated by the introduction into the doped, or active fiber of light pumping energy can occur at several wavelengths, typical of the dopant substance, thereby producing a fluorescence spectrum for the energy emitted by the fiber.
With the object of obtaining the maximum amplification of the signal by means of a fiber of the above type, in combination with a high signal/noise ratio, for optical telecommunications, the transmission signal normally used is generated by a laser emitter with a wavelength corresponding to a peak of the fluorescence spectrum curve of the fiber incorporating the dopant substance used.
In particular, for the amplification of optical telecommunication signals, it is convenient to use active fibers with a core doped with Erbium ions (Er.sup.3+). However, Erbium's fluorescent spectrum, in the range of the wavelengths of interest, has an especially narrow emission peak, and therefore, this imposes the use as the source of the transmission signal of a laser emitter operating at a specific wavelength with a limited tolerance because signals outside such range of tolerance would not be adequately amplified and a strong spontaneous emission signal would occur at such a peak wavelength which constitutes a background noise which would greatly impair the quality of
On the other hand, laser emitters having the above characteristics, that is, operating at Erbium's omission peak, are difficult and costly to manufacture. The usual industrial production provides laser emitters, e.g., semiconductor lasers (In, Ga, As), having several characteristics which make them suitable for use in telecommunications, but have a fairly wide tolerance as to the emission wavelength. Thus, only a limited number of laser emitters commercially produced has the emission at the desired peak wavelength of the dopant.
While, for some applications, such as submarine telecommunication lines, it can be acceptable to use transmission signal emitters operating at a specific wavelength and obtained, by making a careful selection among commercial production lasers so that only those which have the emission within a small range of the laser emission peak of the amplifier fiber, such a process is not financially acceptable for lines of other kinds, such as, urban communication lines, where the containment of installation costs is of special importance.
For example, a fiber doped with Erbium, for providing laser emission, has an emission with a peak around 1536 nm. For a range of about 5 nm from this value, the emission has a high intensity and may be used for the amplification of a signal in the same wavelength range. However, commercially produced semiconductor lasers which may be used for transmission usually have emission wavelength values ranging from 1520 to 1570 nm.
Accordingly, a considerable number of commercially produced lasers of this kind have emission wavelengths outside the range suitable for amplification with Erbium and, therefore, cannot be used for generating telecommunication signals in lines equipped with Erbium amplifiers of the type described hereinbefore.
However, it is known that fibers doped with Erbium have a range of the emission spectrum of high intensity, although lower than the peak, which is substantially constant in the wavelength range contiguous to the above-mentioned peak and is sufficiently wide to include a large part of the emission range of the commercial lasers of interest. In a fiber of this type, a signal provided at a wavelength displaced from the maximum emission peak would be amplified to a limited extent whereas the spontaneous transitions from the laser emission state in the fiber provide emission prevalently at the peak wavelength of the spectrum, at 1536 nm, thereby generating a background noise which will be amplified as it progresses along the length of the fiber, superimposing itself on the useful signal.
In order to use active fibers doped with Erbium for the amplification of telecommunications signal generated by semiconductor laser emitters of a commercial type, the need arises for filtering Erbium's spontaneous peak emission, in the length of the active fiber, so that such emission at an undesired wavelength shall not take pumping energy away from the amplification of the signal and shall not be superimposed on the transmission signal.
For this purpose, an active fiber having two cores can be used in one of which the transmission signal and the pumping energy are present while in the other, there is a light absorbing dopant. If the two cores are optically coupled at the peak wavelength of the spontaneous emission, energy of the spontaneous emission will be transferred to the second core where it is absorbed without returning to one core carrying the transmission signal.
Such an active fiber, described in said U.S. patent application Ser. No. 07/553,246 (U.S. Pat. No. 5,087,108) provides an effective filtering action of the undesired wavelength but in some applications, where the fiber is subjected to mechanical or thermal stresses, and in particular, to twisting, the optical coupling characteristics between the cores can be altered, and the value of the wavelength of the energy transferred to the second absorbing core is modified.
Thus, the problem arises of having available an active optical fiber for use in optical amplifiers which can be used in combination with laser emitters of the transmission signal of a commercial type, without imposing significant qualitative limitations and which is substantially insensitive to deformation stresses and conditions imposed upon it during the construction of the amplifier or occurring during the laying and operational stages of the amplifier in the line.